Minerals removed from the earth, e.g., coal, iron ore and the like, are recovered in two fundamental ways, namely, by underground mining and by surface mining. Surface mining operations use digging machines of a type sometimes referred to as "steam shovels" by persons outside the industry. Of course, such steam-operated machines have not been manufactured for some time.
Surface mining machines are equipped with what are known in the industry as buckets or dippers. In a bucket-equipped machine, the bucket teeth point toward the machine while in a dipper-equipped machine, they point away from the machine. And a dipper has a bottom dump door opened to let the recovered mineral fall from the dipper by gravity. Both buckets and dippers are used as "scoops" to remove the mineral and, sometimes, earth overburden covering the mineral. Such buckets and dippers have a carrying capacity usually measured in cubic yards.
With dipper-equipped machines, it has been known for some time that the ability of the machine to efficiently remove mineral without overloading the machine relates in large part to the relative included angle (sometimes called a "rake" or "attack" angle) measured between the dipper-supporting "handle" or arm and an upward-projected line along the front face of the dipper. To put it another way, adjustment of such angle controls the angle at which the teeth "bite into" the material being removed.
Control of such angle is not a trivial concern, especially on larger machines, and needs to be varied with variations in the material being removed and/or with the size of the dipper used on a particular machine. As an example, a 50.degree. rake angle may be acceptable for soft material or overburden while a 48.degree. rake angle may be better for harder, more dense material.
And for a particular dipper capacity, adjustment of the rake angle can help avoid overloading the machine. This possibility is readily apparent when one considers that coal weighs about 1,500 lbs./cu. yd. while iron ore (which is more than twice as dense) weighs about 3,500 lbs./cu. yd.
In smaller digging machines, selection of the rake angle is not as critical, even with changes in material being mined. However, as the dipper capacity increased with machine size, the rake angle becomes increasingly critical and more sensitive to differing mining parameters. And an experienced operator can detect a difference as small as one-half degree in rake angle.
Conventionally, the rake angle is maintained by a pitch brace extending between the arm and the dipper with the brace, arm and dipper forming a triangle. Selection of the length of the pitch brace (typically a rigid length of steel with a pin "eye" at each end) is sufficiently complex to have been determined using a computer program. However, changes in field requirements sometimes necessitate that the length of the pitch brace be changed. This creates a practical problem.
It is extremely uncommon for a mining crew to have more than the one pitch brace on hand. Mining machines are usually used in remote sites. Replacement parts are difficult to deliver and, because of their size and weight, are even more difficult to install. In other words, replacement of a conventional pitch brace with one of a different length is very time consuming and difficult. There is little margin for error--the length of any needed replacement must be estimated very accurately.
And a mistake in length of a replacement brace is not the only aspect of the problem. A large dipper-type mining machine weighs about 2.5 million pounds and costs several million dollars. When a pitch brace needs to be replaced, the machine is necessarily idled. Such "downtime" is anathema to a mining company.
Until the invention, pitch braces were designed and available only in whole-inch increments. On the other hand, it is not unusual to need an adjustment in brace length on the order of a fraction of an inch to obtain, say, a one-half degree change in rake angle. A more efficient and more precise means of changing the pitch brace length is clearly needed.
As will become apparent, the invention resolves some of these difficulties in unique and imaginative ways.